The present invention relates to a semiconductor device and a fabrication method thereof, and particularly to a semiconductor device which has a capacitor using a high-dielectric-constant or ferroelectric material thin film as a capacitor dielectric film and exhibiting a reduced leakage current, a large capacitance and a high residual polarization, and which is suitable for a memory of a large-scale integrated circuit (LSI), and a fabrication method thereof.
A semiconductor memory, typically a dynamic random access memory (DRAM) has a problem associated with the increased area and complicated structure of a capacitor along with a higher level of integration. To cope with such a problem, examination has been made to use, as a capacitor dielectric film, a high-dielectric-constant or ferroelectric material having a significantly large specific dielectric constant as compared with a prior art capacitor using silicon oxide or silicon nitride. In particular, it has been examined to apply the capacitor dielectric film using the high-dielectric-constant or ferroelectric material to a semiconductor device requiring a large capacitance with a reduced area, such as a large-scale DRAM in which the density of integration is in the order of gigabit. Further, since the high-dielectric-constant or ferroelectric material has a spontaneous polarization, the direction of which can be reversed by an external electric field applied thereto, an attempt has been made to form a non-volatile memory by making use of such a characteristic of the high-dielectric-constant or ferroelectric material. A prior art memory using the high-dielectric-constant or ferroelectric material has been described, for example, in Japanese Patent Laid-open No. Sho 63-201998.
The high-dielectric-constant or ferroelectric thin film used for the above memory is generally made from a composite metal oxide type high-dielectric-constant or ferroelectric material such as lead titanate zirconate (Pb(Zr, Ti)O3) (hereinafter, referred to as xe2x80x9cPZTxe2x80x9d), or barium strontium titanate ((Ba, Sr)TiO3) (hereinafter, referred to as xe2x80x9cBSTxe2x80x9d).
A lower electrode (storage electrode) is generally made from a noble metal excellent in oxidation resistance, such as platinum, iridium or ruthenium for suppressing deterioration thereof due to heating at a crystallization temperature of 500xc2x0 C. or more upon formation of the high-dielectric-constant or ferroelectric film. On the other hand, an upper electrode (plate electrode), which is usually formed after film formation of a composite metal oxide, is generally made from the same material as that of the lower electrode for enhancing an electrical symmetry, and to avoid reaction with the high-dielectric-constant or ferroelectric film at a heat-treatment step performed after formation of a capacitor, the upper electrode is often made from a noble metal such as platinum (for example, described in the specification of U.S. Pat. No. 5,005,102).
In the fabrication process for the above memory, it however, since the process has a processing step carried out in a hydrogen atmosphere such as a step of forming an interlayer insulating film, if the electrode is made from an electrode material acting as a strong catalyst to reduction such as platinum, the oxide type high-dielectric-constant or ferroelectric material is reduced. This causes a significant deterioration in characteristics, such as the increase in leakage current or disappearance of the hysteresis characteristic.
For this reason, a process not to generate hydrogen has been selected at the sacrifice of the characteristic of an interlayer insulating film to some extent. However, it may be of course desirable to use a technique of forming an insulating film being good in coverage and resistance against etching. Further, even at the step of forming an interconnection after formation of a capacitor, hydrogen may be used, and to increase the degree of freedom in selection of the process, the capacitor using a high-dielectric-constant or ferroelectric film is required to be improved in terms of the resistance against hydrogen heat-treatment.
To be more specific, as described above, after formation of a high-dielectric-constant or ferroelectric film, the capacitor is subjected to treatment in a reducing atmosphere for forming an interconnection layer and an insulating film. Further, a through-hole formed for electrical connection between a peripheral circuit and an interconnection layer is generally formed into a shape in which a ratio of the depth to the size of the opening, that is, a so-called aspect ratio is large, and accordingly, tungsten or the like is formed by a CVD process in such a manner as to bury the through-hole. The CVD process is also performed in a reducing atmosphere.
It is known that the capacitor suffers a serious damage when being subjected to the above-described treatment in a reducing atmosphere. For example, in Material Research Society Symposium Proceedings Vol. 310, pp. 151-156 (1993), it was reported that when a SiO2 film is formed by CVD, PZT as a high-dielectric-constant or ferroelectric material loses its ferroelectricity and increases the leakage current. Further, in the memory fabrication process, the memory is finally subjected to hydrogen heat-treatment (hydrogen annealing) for ensuring the reliabilities of a metal interconnection layer and a transistor formed in a layer under the capacitor. It is known that like the above step of forming an interlayer insulating film, the hydrogen annealing exerts an effect on the capacitor characteristics. For example, in the eighth International Symposium on Integrated Ferroelectrics, 11C (1996), it was reported that in the case of using SrBi2Ta2O3 (hereinafter, referred to as SBT) as a high-dielectric-constant or ferroelectric material, when being subjected to treatment in a hydrogen atmosphere, the capacitor is peeled, or if not peeled, it causes significant deterioration in leakage current characteristic.
(Solving Means)
To solve the above-described problems, according to the present invention, there is provided a high-dielectric-constant or ferroelectric capacitor including an upper electrode formed of a conductive film of iridium oxide or ruthenium oxide, characterized in that lead, bismuth or barium is added to iridium oxide or ruthenium oxide for reducing the catalytic action of iridium or ruthenium which is not oxidized and remains.
It was examined what kind of material should be used as an upper electrode for a high-dielectric-constant or ferroelectric capacitor capable of keeping its characteristics after the step in a hydrogen atmosphere. A double-layer film of platinum and titanium was formed as a lower electrode on a silicon substrate on which a thermally oxidized film was formed. A lead titanate zirconate thin film having a thickness of 100 nm was formed on the electrode by a sol-gel process. The sol used was obtained by reaction of lead acetate, titanium isopropoxide, and zirconium isopropoxide in methoxy ethanol. To obtain a perovskite type structure upon crystallization, 10% of lead oxide was excessively added thereto. The lead titanate zirconate thin film was subjected to rapid thermal annealing in an oxygen atmosphere at 650xc2x0 C. for 2 minutes, to crystallize lead titanate zirconate. An upper electrode having a size of 100 xcexcmxc3x97100 xcexcm was formed on the lead titanate zirconate thin film by a lift-off process. The capacitor with upper electrode thus obtained was heat-treated in a hydrogen atmosphere at 300xc2x0 C., and was examined in terms of the presence or absence of deterioration of the characteristics. To be more specific, four kinds of the capacitors including the upper electrodes made from platinum, iridium oxide, stacked layer of platinum and iridium oxide, and gold were examined. A ratio of the spontaneous polarization value after hydrogen heat-treatment to that before hydrogen heat-treatment for each capacitor was shown in Table 1.
As is apparent from Table 1, the deterioration of the characteristic of the capacitor is dependent on the kind of the upper electrode material, more specifically, becomes smaller in the order of platinum, stacked layer of platinum and iridium oxide, gold, and iridium oxide, and for the capacitor including the upper electrode made from iridium oxide, there is no deterioration of the characteristic shown.
From the results shown in Table 1, it was revealed that if the material having acted as a catalyst to reduction by hydrogen is used as the upper electrode, it causes the deterioration of the characteristic, and even if the material is not direct-contact with the lead titanate zirconate thin film like the stacked layer of platinum and iridium oxide, it promotes the deterioration of the characteristic. With respect to the deterioration of the characteristic caused in the case of using gold as the upper electrode, it was revealed that since such deterioration is substantially equal to that caused by heat-treatment in a nitrogen atmosphere at the same temperature, it is not due to reduction by hydrogen but is only the thermal deterioration.
In summary, the degree of deterioration by hydrogen heat-treatment is largely dependent on the kind of the upper electrode material, and by using iridium oxide which is a conductive oxide as the upper electrode, the deterioration can be effectively suppressed.
The above hydrogen heat-treatment was performed at 300xc2x0 C. The test was then repeated by increasing the temperature of the hydrogen heat-treatment. As a result, it was confirmed that even the capacitor including the upper electrode made from iridium oxide was slightly deteriorated. This is because the reduction of iridium oxide begins by heat-treatment at a high temperature so that the catalytic action of iridium oxide emerges. Next, the upper electrode made from iridium oxide was formed and was heat-treated in an oxygen atmosphere, and then examined in terms of the deterioration preventive effect. The effect obtained by oxygen heat-treatment performed before hydrogen heat-treatment was shown in FIG. 1. As is apparent from FIG. 1, by previously heat-treating the upper electrode made from iridium oxide in an oxygen atmosphere at a temperature of 500xc2x0 C. or more, the deterioration by hydrogen heat-treatment is effectively suppressed.
To clarify the effect obtained by oxygen heat-treatment, the electrode portion was measured by X-ray diffraction and X-ray photo-electron spectroscopy. The X-ray diffraction patterns of the electrode portion before and after oxygen heat-treatment were shown in FIG. 2. As is apparent from FIG. 2, the crystallinity of the iridium oxide film becomes improved with an increase in the oxygen heat-treatment temperature. Also from the result of the X-ray photo-electron spectroscopy, it was revealed that lead is diffused in the electrode made from iridium oxide. FIG. 3 shows the dependency of the oxygen heat-treatment temperature on the diffused amount of lead. As is apparent from FIG. 3, when the oxygen heat-treatment temperature becomes 500xc2x0 C. or more, the diffusion of lead is rapidly emerged. It became apparent that the effect obtained by oxygen heat-treatment is that as the degree of oxidation of iridium oxide is increased, the reduction of iridium oxide upon hydrogen heat-treatment is suppressed by lead diffused from the lead titanate zirconate thin film, to thereby prevent the emergence of the catalytic action and the deterioration of the capacitor characteristics due to the emergence of the catalytic action.
Next, the additional effect of lead to the electrode made from iridium oxide was further examined. A lead added iridium oxide film was formed as the upper electrode by a manner of placing a desired amount of pellets of lead on an iridium metal target, and sputtering the composite target by a reactive sputtering process. FIG. 4 shows a relationship between the added amount of lead in the iridium oxide film (molar fraction of lead based on the iridium oxide) and the deterioration of the capacitor characteristics at the hydrogen heat-treatment temperature. As is apparent from FIG. 4, the deterioration suppressing effect is improved by adding lead even in an extremely small amount. When the added amount of lead reaches in a range of less than 10%, the effect is improved, however, when it is more than 10%, the effect is saturated and the resistance of the electrode portion is increased. Accordingly, the added amount of lead is preferably in a range of 10 mol % or less.
The same effect was obtained by a manner of forming the iridium oxide film and stacking a lead film thereon for subjecting to heat-treatment. In the method, the outermost surface portion may be lead oxide, however, in this case, the surface may be subjected to sputter-etching to remove the lead oxide.
While the above effect was obtained for the capacitor in which lead titanate zirconate was used for the high-dielectric-constant or ferroelectric thin film and iridium oxide was used for the upper electrode, the same effect was obtained for a capacitor in which bismuth layered high-dielectric-constant or ferroelectric material was used for the high-dielectric-constant or ferroelectric thin film. As the element to be added in iridium oxide, bismuth may be used in place of lead. Further, the same effect can be obtained for a capacitor in which barium strontium titanate was used for the high-dielectric-constant or ferroelectric film and ruthenium oxide was used for the upper electrode. Such a capacitor is easy to be applied to a DRAM because it does not exhibit the hysteresis characteristic at room temperature although it depends on the composition. Although the heat-treatment in a hydrogen atmosphere causes a problem in increasing the leakage current, it is possible to suppress an increase in leakage current by adding an element capable of making the reduction by hydrogen smaller. As the additional element in this case, barium may be used in place of lead.
Further, according to the present invention, an impurity is added in an upper electrode (electrode formed after formation of a high-dielectric-constant or ferroelectric film) of a capacitor, to make a hydrogen decomposition action of the electrode metal small. The additional element is desirable to be small in solubility in the metal used for the electrode. To be more specific, in the case of using platinum (Pt) as the upper electrode metal, examples of the desirable additional impurity elements may include sulfur (S), selenium (Se), tellurium (Te), silicon (Si), boron (B), phosphorus (P), arsenic (As), and bismuth (Bi).
In the case of the upper electrode made from a metal other than platinum or gold, the same effect can be obtained by adding lead (Pb) or barium (Ba) in place of the above-described impurity element. For example, in the case of the upper electrode made from any one of palladium, ruthenium, iridium and nickel in place of platinum, the same effect can be obtained by adding the above impurity.
Table 2 shows the solubility in platinum of each of the above elements, and compounds containing platinum in the largest amount amongst other compounds of platinum (the source: Binary Alloy Phase Diagrams, 2nd Ed., Thaddeus B. Massalski, Editor-in-chief, ASM International, 1990).
The solubility in platinum of each of the above additional elements is less than 10%. Accordingly, when platinum is crystallized into a polycrystalline state at the step of depositing a platinum layer or at the step of heat-treating the platinum layer after depositing step, most of the additional element are turned into a state near the compound shown on the right column of Table 2 and they cover the surfaces of polycrystalline grains of platinum. The catalytic activity on the surface of platinum is thus lowered. As a result, upon heat-treatment in a hydrogen atmosphere, occurrence of active hydrogen due to decomposition of hydrogen on the surface of platinum is suppressed, and thereby the deterioration of the capacitor characteristics and the peeling of the electrode are also suppressed. Further, since the concentration of the above impurity in platinum is small, there is no deterioration of the element characteristics due to an increase in electrical resistance of the entire platinum layer.
Even in the case of using, as the upper electrode material, palladium, ruthenium, iridium or nickel in place of platinum, the same effect can be obtained by adding the above impurity. It is known that of the above additional impurities, sulfur is an element of lowering the catalytic activity of platinum and palladium (described, for example, in H. P. Bonzel and R. Ku, The Journal of Chemical Physics, Volume 58, Number 10, page 4617-4624 (1973) and Y. Matsumoto et. al., Journal of Chemical Society Faraday I, Volume 76, page 1116-1121 (1980)). According to the present invention, the deterioration of the characteristics of the high-dielectric-constant or ferroelectric capacitor was prevented by positively utilizing the above effect.
A desirable amount of the above additional element will be described by referring to an example of platinum. In general, a platinum thin film used for the electrode is in a columnar polycrystalline state. It is assumed that a polycrystal grain of platinum has the shape of a column having a radius xe2x80x9crxe2x80x9d and a height xe2x80x9chxe2x80x9d; and the face density of the platinum atom on the surface of the column is equal to the value of the (100) face of the platinum crystal, which is taken as 2/a2. The volume density of the platinum atom in the column is 4/a3. The symbol xe2x80x9caxe2x80x9d designates the lattice constant of the platinum crystal, which is 0.39 nm. On the above assumption, a ratio (s) of the number of the platinum atoms exposed on the surface of the column to the number of the platinum atoms in the entire column is given by an equation of (r+h)a/r/h. The height of the column may be regarded as a value nearly equal to the thickness of the platinum film,which is generally about 100 nm. The radius is generally 10 nm or more. Assuming that h=100 nm and r=10 nm, Sxe2x88x924 atom % is obtained. Accordingly, by adding the element in an amount of several atom %, the element can sufficiently cover the platinum atoms on the surfaces of the polycrystal grains of platinum. It is not desirable to increase the amount of the additional element more than necessity since the electrical resistance of the platinum electrode is undesirably increased. Further, the additional element in an excessive amount may be diffused in the high-dielectric-constant or ferroelectric thin film to cause the deterioration of the dielectric characteristic or may be diffused in the insulating protective film to cause the deterioration of the insulating characteristic. These problems can be avoided by setting the added amount of the additional element in a range of 10 atom % or less. The same is true for the upper electrode made from any one of palladium, ruthenium, iridium, and nickel.
The present invention is intended to suppress the deterioration of the capacitor due to treatment in a hydrogen atmosphere which is performed after the upper electrode is formed to accomplish the capacitor, and therefore, it is essential to apply the present invention to the upper electrode of the two electrodes (lower electrode and upper electrode) constituting the capacitor. Since the permeation of hydrogen in the lower electrode is suppressed by the high-dielectric-constant or ferroelectric layer and the upper electrode, the advantage obtained by applying the present invention to the lower electrode is smaller than that obtained by applying the present invention to the upper electrode. In some cases, it may be rather desirable to use a metal layer to which the impurity is not added for the lower electrode for preventing the electrical resistance and the contact resistance with the underlying polysilicon layer from being increased due to the addition of the impurity.
As a result of the search for any known example on the basis of the above viewpoint, Japanese Patent Laid-open No. Hei 4-206871 was found. This document discloses a semiconductor device at least including a first electrode, a high-dielectric-constant or ferroelectric film, and a second electrode sequentially stacked on a substrate, characterized in that at least one of the first and second electrodes is composed of a platinum or gold electrode containing at least one kind selected from a group consisting of lead, barium, lanthanum, strontium, titanium and zirconium. In this technique, the same elements as those contained in the high-dielectric-constant or ferroelectric material are previously contained in the lower electrode for preventing occurrence of an inconvenience that the components of the high-dielectric-constant or ferroelectric material are diffused in Pt or Au constituting the lower electrode upon formation of the high-dielectric-constant or ferroelectric film, causing change in the composition of the high-dielectric-constant or ferroelectric material, thereby lowering the dielectric constant. The above document also describes that the elements may be contained even in the second electrode, however, it does not describe the problem caused in the case of using Pt for the upper electrode at all.
To fabricate a semiconductor device, hydrogen annealing is an essential process, however, it causes a problem in exerting adverse effect on the capacitor characteristics. Accordingly, it is important to solve such the problem for obtaining a semiconductor memory, particularly, at the level of gigabit or more.
As a result of examining the cause of deterioration due to treatment in a hydrogen atmosphere, it was revealed that platinum as the electrode material is concerned with the deterioration process.
A relationship between the state of the interface between Pt as the electrode material and PZT and the influence of hydrogen (H2) annealing was examined by XPS (X-ray photoemission spectroscopy). A PZT thin film was prepared by ozone jet evaporation (OJE) in an ozone atmosphere. In a chamber connected to an analytical chamber, a Pt thin film was formed on the PZT thin film by electron beam vapor-deposition and was annealed, and the state of the interface between Pt and PZT of the Pt/PZT sample was subjected to in-vacuo XPS analysis. As a result, it was revealed that metal Pb is produced at the interface by annealing at 300xc2x0 C.; the amount of metal Pb becomes larger when the Pt/PZT sample is subjected to H2 (0.5 Torr) annealing; and the n-type Schottky barrier height at the Pt/PZT interface is lowered by about 0.6 V when the Pt/PZT sample is subjected to H2 annealing (FIG. 20 shows a change in Pb4fXP spectrum of the PT/PZT sample, depending on annealing (300xc2x0 C., 20 minutes)). The reason for this may be considered that H radicals produced by dissociation/absorption of H2 on the surface of Pt strongly acts on the surface of PZT, with a result that the interfacial level due to oxygen vacancies is generated and thereby the Schottky barrier is lowered.
To be more specific, in the case of using platinum (Pt) as the electrode material, hydrogen is decomposed on the surfaces of polycrystal grains of Pt, and active hydrogen is produced by the catalytic action of Pt. The active hydrogen deteriorates the high-dielectric-constant or ferroelectric material. This is the reason why the capacitor characteristics are deteriorated and the electrode is peeled at such a low temperature (for example 300xc2x0 C.) at which the high-dielectric-constant or ferroelectric material is not generally reduced to be deteriorated.
As a result, it is required to eliminate the catalytic activity of Pt for preventing the deterioration of the Pt/PZT interface due to H2 annealing and occurrence of damages of the high-dielectric-constant or ferroelectric film due to hydrogen.
(Effect)
As described above, for an oxide high-dielectric-constant or ferroelectric capacitor using an electrode made from platinum, hydrogen dissociated from a raw material at the passivation step becomes active by the catalytic action of the electrode and reduces the high-dielectric-constant or ferroelectric material, thereby deteriorating the capacitor characteristic. A conductive iridium oxide layer used in the present invention, however, is weak in terms of catalytic action, and therefore, it does not make hydrogen active to reduce the high-dielectric-constant or ferroelectric material. Further, an additional element and its oxide function to suppress emergence of the catalytic action of the electrode due to reduction.
Even in the case of using platinum or the like as the material of an upper electrode, a desirable result can be obtained by adding a suitable impurity such as sulfur (S).